1. Field of the Invention
The present invention relates to thread gaging generally and, more particularly, but not by way of limitation, to a novel thread gage and method that permit rapid and convenient measurement of the parameters of a thread.
2. Background Art
A screw thread is a helical surface formed on a member and two such surfaces are commonly employed for joining members, typically intersection of a pair of internally and externally complementarily threaded cylindrical members. Similar surfaces are provided on gears for the transmission of motion between two or more gears.
Depending on the application, the dimensional accuracy with which such threads are formed is more or less critical. Over time, standards have evolved for measuring the dimensional accuracy of threads. See, for example, "Gages and Gaging for Unified Inch Screw Threads," ANSI/ASME B1.2-1983, published by The American Society of Mechanical Engineers, New York, N.Y., which document and the references cited therein are made a part hereof by incorporation by reference hereinto.
The primary measurement applied to straight screw threads is that of "pitch diameter" which is commonly defined as the diameter of an imaginary cylinder, the surface of which would pass through the threads at such points as to make equal the width of the threads and the width of the spaces cut by the surface of the cylinder. Conventional methods of making this measurement involve the contacting of the flanks of opposing threads with a "three-wire" gage, a "best-wire-size" gage, or "cone and vee" gages. A substantial limitation of these devices is that they, and variations thereof, contact the flanks of the threads at single points or at narrow lengths thereof. Consequently, if a thread had the appropriate dimension at the points of contact, but were otherwise greatly misshapen, the thread would pass inspection without the defects being discovered. Misshapenness in terms of excess metal on the flanks of the thread may be discovered through the use of a "full-form" gage roll which substantially fills the entire thread, but this type of gage will not ordinarily discover many types of misshapenness where there is missing metal. The only known satisfactory way to fully analyze the true shape of the flanks of a thread is through the use of optical imaging devices. These are somewhat cumbersome and require a separate operation from the standard thread gaging operation.
Another parameter of importance in measuring the accuracy of the forming of thread is that of lead "stretch" or "shrink", that is, a variation in thread spacing that is greater or less, respectively, than desired. This parameter is typically measured through the use of a multiple-rib, full-form gage roll which inserts teeth in a plurality of grooves of the thread. Comparing this depth to that of a single full form rib indicates that there is lead stretch, shrink, or thread taper, but it is not possible to determine which exists from the measurement and, consequently, further investigation must be made to determine whether stretch or shrink lead or taper is present.
A further imperfection in threads is a drunk thread in which a portion of a single turn of the thread is of greater or less thickness than it should be. Again, conventional thread gaging methods cannot easily distinguish this imperfection from the others noted above.
Accordingly, it is a principal object of the present invention to provide means and method for gaging threads that can readily detect variations in the shape of the flanks of the threads.
It is an additional object of the present invention to provide means and method for gaging threads that can readily detect whether threads have stretch or shrink lead or taper.
It is a further object of the invention to provide such means and method for gaging threads that can readily detect drunk threads.
It is another object of the invention to provide such means and method that are rapidly and conveniently employed.
Other objects of the present invention, as well as particular features, elements, and advantages thereof, will be elucidated in, or be apparent from, the following description and the accompanying drawing figures.